1. Field of the Invention
The present invention relates generally to a light attenuating element employable for an optical fiber communication circuit system. More particularly, the present invention relates to a light attenuating element preferably employable for a stationary type optical attenuator adapted to be optically connected to an optical connector widely used in optical fiber communication circuit systems. Further, the present invention relates to a method of producing a light attenuation element of the foregoing type.
2. Description of the Related Art
Various kinds of stationary type optical attenuators have been heretofore put in practical use for the purpose of attenuating a quantity of light beam to pass through an optical fiber communication circuit network.
Among many stationary type optical attenuators, an optical connector type optical attenuator has advantages in that it can be easily handled, and moreover, it can be used within a wide range of application fields. For this reason, optical connector type optical attenuators have been hitherto widely used in many application fields.
To facilitate understanding of the present invention, typical conventional optical attenuators will briefly be described below with reference to FIGS. 5(a) and 5(b).
FIG. 5(a) is a sectional view of a conventional stationary optical attenuator, particularly illustrating the structure of the optical attenuator. The optical attenuator includes ferrules 45 and 46, and optical fibers 43 and 44 with sheathes 41 and 42 removed therefrom inserted through center holes of the ferrules 45 and 46. After completion of the inserting operation, the optical fibers 43 and 44 are immovably held in the center holes of the ferrules 45 and 46 with the aid of an adhesive. The outer end surfaces of the ferrules 45 and 46 are ground and polished together with the optical fibers 43 and 44 to assume a predetermined contour. Then, the ferrules 45 and 46 are inserted into an alignment sleeve 47 from the opposite sides of the optical attenuator, until an end surface 48 of the ferrule 45 comes in contact with an end surface 49 of the ferrule 46, whereby a series of optical signals can be transmitted through the optical fibers 43 and 44.
Next, FIG. 5(b) is a sectional view of another conventional stationary optical attenuator, particularly illustrating the structure of the optical attenuator. This optical attenuator is substantially the same as that shown in FIG. 5(a) with the exception that a glass plate 50 is interposed between both the ferrules 45 and 46. The opposite surfaces of the glass plate 50 are coated with a film having a vaporized metal deposited thereon so as to allow a predetermined light attenuation value to be obtained. Specifically, the ferrules 45 and 46 are inserted into the alignment sleeve 47 from the opposite sides of the optical attenuator such that their foremost ends come in contact with the glass plate 50 (see Light Guide Digest, Vol. 2, 1990 published by AT & T). In practice, this type of stationary optical attenuators have been hitherto mainly employed to build an optical fiber communication circuit network.
However, since a glass plate having surfaces each plated with a vaporized metal is used in the conventional optical attenuator, a light attenuation value of the optical attenuator varies depending on the thickness of each plated layer, and it is practically difficult to exactly control the thickness of each plated layer so as to obtain a specified light attenuation value. For this reason, the number of inspection steps is unavoidably increased to detect an incorrectly plated layer. Thus, when economical conditions are taken into consideration, a commercially available allowance of light attenuation value is restrictively defined to remain within the range of about .+-.1.5 dB. However, this level is not satisfactory and acceptable from the viewpoint of accuracy in some application fields.
In a case where ferrules are frequently attached to and detached from the alignment sleeve during practical use, there arises a malfunction that the plated layer present in the contact location is peeled from the glass substrate. Once peeling occurs, it is impossible to repair the glass plate in the optical attenuator.
Another drawback of the conventional optical attenuator is that a malfunction of so-called reflected return light loss readily arises when an optical signal is reflected at a connection plane where two optical fibers are optically connected to each other and then the optical signal returns in the direction toward the light source. However, this reflected return light loss is very harmful for a wide range optical fiber communication system such as an ISDN communication system which has been researched and developed at present. To assure that the wide range optical fiber communication system is actually realized, it is necessary that the reflected return light loss can be reduced remarkably.
In view of the fact that the reflected return light loss increases in proportion to the number of connection locations, it is inevitably essential to avoid as far as possible the interposition of a glass plate having surfaces each plated with a vaporized metal between the ferrules.